Layered films of metal-insulator-metal are employed as storage elements in memory devices such as magnetic random access memories (MRAM) and the like. The memory element for the MRAM technology is a patterned structure of multilayer material and is usually composed of a stack of different materials such as NiFe, CoFe, PtMn, Ru, etc., and may include insulator-like materials such as Al2O3 or MgO. A typical stack may contain as many as ten or more layers of these materials some of which are non-magnetic, some of which are magnetic, and one or two of which are insulating. The insulating films in this description are defined as oxidized or nitridized metal layers that exhibit high electrical resistance in their bulk form. To fabricate a storage element, it is necessary to deposit the materials in overlying blanket films, layer by layer, to form a patterned layer of photoresist, and to etch the films into appropriate structures.
Ion beam milling or ion bean etching processes have been employed to remove magnetoresistive materials. Ion beam milling, however, is a physical milling process. Areas that are not protected by the mask are removed by bombardment with ions. The bombardment of ions sputters or peels away the unprotected material. Ion beam milling operates with low selectivity, and the portions of the stack that are near to the edges of the mask or the boundaries of an MRAM cell body can be easily damaged.
Chemical etching techniques have also been employed to selectively remove portions of deposited layers. Examples of etching techniques include dry etching techniques and wet etching techniques.
One of the drawbacks of current etching techniques is that the profiles of MRAM structures are susceptible to electrical shorting across the thin tunnel junction. The vertical separation between the upper magnet layer above the insulating dielectric tunneling layer and the lower magnet layer below this tunneling layer is inadequate to prevent electrical shorting.